1. Field of the Invention
The present invention relates to a backlight unit using as a backlight for a liquid crystal panel or the like, and particularly to a backlight unit of the so-called side light or edge light system in which light from a light-emitting body is introduced through a side surface of a light guide plate.
2. Description of the Related Art
Hitherto, a liquid crystal display device has been known in which a backlight unit in the same shape as a transmission type liquid crystal panel is disposed on the back side of and in parallel to the liquid crystal panel, and light is radiated from the rear side of the liquid crystal panel by the backlight unit. The liquid crystal panel is constituted, for example, by sealing a liquid crystal between transparent electrode substrates mutually laminated with a seal, a spacer or the like therebetween, and polarizer plates are adhered respectively to the transparent electrode substrates.
The backlight unit includes a light diffuser plate, a light guide plate, and a reflector plate in this order from the side of the liquid crystal panel, and a fluorescent tube as a backlight light source is disposed along a side surface of the light guide plate. Further, a reflector film is fitted to the light guide plate so as to cover the fluorescent tube, whereby the light from the fluorescent tube is efficiently introduced into the light guide plate through the side surface of the light guide plate by the reflector film.
In the liquid crystal display device having such a structure, first, the light emitted directly from the fluorescent tube and the light reflected by the reflector film are incident on the light guide plate through the side surface of the light guide plate. The light incident on the light guide plate repeatedly undergoes multiple reflection inside the light guide plate and on the reflector plate provided on the back surface of the light guide plate, and thereafter comes out through the front surface of the light guide plate. The light coming out of the light guide plate is diffused by the light diffuser plate, and is incident on the liquid crystal panel.
In such a backlight unit, when it is intended to obtain a higher luminance, the amount of heat generated at electrode portions of the fluorescent tube is enlarged, resulting in a temperature in excess of 80 to 100xc2x0 C. As a result, there are the problems that bad effects are exerted on the reflector film, the light guide plate or a casing of the backlight unit and the like, and nonunifornmity of picture is generated in the liquid crystal panel due to the thermal effects.
In order to solve the problems, Japanese Patent Laid-open No. Hei 10-106329 discloses a backlight unit in which heat-radiating sheets formed of copper foil are adhered to electrode portions of the fluorescent tube for the purpose of cooling (heat radiation) at the electrode portions of the fluorescent tube. In this backlight unit, since the heat-radiating sheets are adhered to the electrode portions of the fluorescent tube, a certain extent of heat radiation effect can be expected. However, soldered portions between the terminals of the fluorescent tube and leads are not in direct contact with the heat-radiating sheets, respectively, so that there is a problem that the heat radiation effect at the soldered portions is insufficient, and the solder is deteriorated due to a rise in temperature. When the temperature at the soldered portions is raised to about 100xc2x0 C., a tensile stress is exerted on the soldered portions due to thermal expansion, so that the soldered portions are deteriorated, leading to contact failure between the terminal and the lead.
Another heat radiation structure for a liquid crystal display device including a backlight light source is disclosed in Japanese Patent Laid-open No. Hei 11-24072. The liquid crystal display device described in the publication has a structure in which a heat-radiating plate is so disposed that a clearance is formed on the back side of a light guide plate, and the clearance is so set as to be smaller as the distance from a fluorescent tube disposed along a side surface of the light guide plate increases, whereby nonuniformity in temperature of a liquid crystal panel is suppressed, and nonuniformity in display is prevented. However, even in the liquid crystal display device described in the publication, nothing is mentioned as to the deterioration of soldered portions between terminals of the fluorescent tube and leads arising from heat generation. Therefore, there is the fear that the soldered portions between the terminals of the fluorescent tube and the leads may be deteriorated due to heat generation, and contact failure between the terminal of the fluorescent tube and the lead may occur.
Accordingly, it is an object of the present invention to provide a backlight unit in which deterioration of soldered portions between terminals of a fluorescent tube and leads due to heat generation is prevented from occurring and contact failure between the terminal and the lead is prevented from occurring.
In accordance with an aspect of the present invention, there is provided a backlight unit including a light guide plate, a fluorescent tube having terminals at both ends thereof and disposed along a side surface of the light guide plate, a reflector plate mounted to a one surface of the light guide plate, a reflector film mounted to the light guide plate so as to cover the fluorescent tube, a pair of leads soldered respectively to the terminals, and a heat-radiating plate provided at at least one of soldered portions between the terminals and the leads.
Preferably, the heat-radiating plate is co-soldered to the soldered portion between the terminal and the lead. By this, the heat generated at an electrode portion of the fluorescent tube is transferred to the terminal, the solder and the heat-radiating plate by heat conduction, and is radiated from the heat-radiating plate. As a result, a temperature rise at the soldered portion can, be restrained, and deterioration of the soldered portion can be prevented.
In accordance with another aspect of the present invention, there is provided a liquid crystal display device including a liquid crystal panel, a light guide plate mounted to a one surface of the liquid crystal panel, a fluorescent tube having terminals at both ends thereof and disposed along a side surface of the light guide plate, a reflector plate mounted to a principal surface of the light guide plate on the side opposite to the liquid crystal panel, a reflector film mounted to the light guide plate so as to cover the fluorescent tube, a pair of leads soldered respectively to the terminals, and a heat-radiating plate provided at at least one of soldered portions between the terminals and the leads.
The above and other objects, features and advantages of the present invention and the manner of realizing them will become more apparent, and the invention itself will best be understood from a study of the following description and appended claims with reference to the attached drawings showing some preferred embodiments of the invention.